In your code, you have the font-family applied to the body tag, so yes of course it will be applied to the entire page, because the whole page is wrapped in <body></body> tags.
There are two different ways to correctly apply the “Rock Salt” font to only a specific element.
Option 1, add the styling inline in your HTML like so:
<p style="font-family:'Rock Salt';font-size:49px;color:#F8B509;text-align:center;">Explore Your Truth</p>
or Option 2, add CSS with a unique class name (“my-class-name” in this example):
<style>
.my-class-name {
font-family:'Rock Salt';
font-size:49px;
color:#F8B509;
text-align:center;
}
</style>and then add the class name to your HTML like so:
<p class="my-class-name">Explore Your Truth</p>